It has been recognized that there is a need to monitor the existence and effects of damage and corrosion in structural materials. Aircraft components provide a much publicized example, but the need exists in a variety of other structures, for example pipelines and building members. To detect defects, traditional modal analysis testing generally relies on structural excitation and vibration pickups. Structural excitation methods, in turn, usually employ either harmonic sweep or impulse excitation. The former method is more precise and can focus on resonant frequencies, while the latter is more expedient. Vibration pickups can measure displacement, velocity or acceleration. The use of self-conditioning accelerometers and laser velocimeters are known. Accelerometers allow installation of sensor arrays that accurately and efficiently measure mode shapes, while lasers offer contactless measurements that are effective for low mass sensitive structures.
In general, it is known to propagate electromagnetic signals through materials in order to measure anomalies in the materials. Ultrasonic transducers, for example, include piezoelectric crystals of a certain mass. The crystals are excited at a known resonant frequency so that the resonant crystal vibration generates an ultrasonic signal. Ultrasonic sensors, which are relatively bulky and expensive, are typically used in portable devices placed in communication with the structure of interest so that the crystals propagate ultrasonic waves into the material. Sensors detecting resulting signals in the material sense patterns that indicate anomalies in the materials such as cracks or corrosion.